An Au where some downworlders and nephilims are closer to their instincts(angel/demon side) than others and can have quite...gory or scary courting methods.
Magnus Bane was known for having many people falling in love with him but had only ever been in two relationships(Camille and another that is up to the writers choice) as by rumours it is said he has high standards.
In truth, Magnus was very close to his demon/fallen angel instincts and his 'high standards' were simply too horrifying for others to the point that while he had been in two relationships, those he had been with hadn't completely been in agreement with how he was and he changed himself for them.
While Magnus knew that there were others like him, none of them seemed to fit him right so he had almost given up to find someone who understood him and could love him completely until one night, a shadowhunter came to his door with a creature's body that was still bleeding and dirtying the hallway.
He had been confused until said shadowhunter said he wished to court magnus and hoped that the creature that had been giving problems to warlock children would be a good first courting gift.
Magnus was surprised by this but was also moved by it as no one had given him such wonderful gift not even his best friend who while respecting his taste, kept way from it more often than not.
Of course that Magnus didn't simply accept immediately despite how enamoured by the gift he was, he demanded to know the shadowhunter's name and what made him wish to court Magnus
The shadowhunter presented himself as Alexander 'Alec' Lightwood, the new Head of New York's Institute and son of two people that most of the downworld in New York hated.
That almost made Magnus take a step back but he decided to keep listening as he knew that the children aren't always like their parents.
Alec then went on to say he had seen Magnus when he was a teen as the warlock came to take care of the wards of the Institute and Alec found him interesting so he decided to ask people about him but once seeing that most people told him what they believed to be the truth, Alec decided to investigate about Magnus himself.
He said he found himself being even more interested on him which didn't change as he grew up so he planned to court Magnus once he had reached the same level as Magnus in power(so yes, Alec would have more power inside the Clave then he did in canon).
Magnus found himself fascinated even more and decided to give Alec a chance to court him and see where it would lead.
Each of Alec's courting gifts were perfect, just bloody enough but with a good reason behind it, so Magnus couldn't wait for the last courting gift and if he would still be given such beautiful gifts once they got married(this all happened before Clary appeared).
But before Alec could prepare everything he needed to for the final courting gift, his siblings and Raj or other shadowhunter(because Alec while protective of his siblings, he won't be allowing them to do whatever they wish and walk all over him) brought a mundane to the Institute and runed her without knowing if she might actually have enough angel blood to take runes and instead of calling a warlock to save her.
Of course that he gave a very stern talk to the three shadowhunters and put them in ichor duty besides forcing them to take classes with the little kids seeing that didn't seem to understand what could have happened.
When Clary woke up, Alec was the one there and he put her down in her place when she started to make demands and explained to her how dangerous their world was and while they could take her commission to find her mother, if she wanted to help she would have to train like any other shadowhunter and only after a very throughout evaluation would she be allowed to help with missions.
Clary seemed to accept it after having some truths thrown to her face(like, she was raised mundane but she didn't care about how dangerous the shadowworld is? Like, people died, demons targeted people to kill or have sex with them, there were fights between different species, etc and she simply didn't care about it? I know she loved her mother but still, she should have taken a moment before bulldoze over everything and everyone) but unfortunately Jace and Izzy didn't seem to have listened to Alec and decided to take Clary to the Silent City and when she asked them if Alec let them, they said he did(which was a lie).
While it brought them the news that Clary parents were Valentine and Jocelyn and that her memories were taken by a warlock, Alec had been furious with his siblings and grounded them to their rooms with no permission to leave them as someone would take food to them.
Alec decided to talk again with Clary and ask if she wished to retrieve her memories, which may or not help her find her mother, before they started her training and Clary said that even if they didn't help her, it might give her information that may help her adjust to the shadowworld.
With that Alec called Magnus and told him about Clary to which the warlock told him everything he knew and that he needed to summon a demon to give back Clary's memories so Alec took Clary to Magnus were he had been having snacks with his friends(Ragnor, Catarina and Raphael) who agreed to help with the summoning of the demon.
When this was done, Alec brought Clary back and put her under the care and teaching of who had been Alec's own teacher(not Hodge) before he went back to planning things for his last gift to Magnus.
And said gift came to to his hands much earlier when Jace and Izzy were not grounded anymore and attracted Camille Belacourt's attention to certain mundane which she kidnapped.
Alec grounded his siblings again and went himself to get Simon back after reassuring Clary that he would bring him alive.
He talked with Raphael, who met him at the door, and told him that he would only punish Camille and leave the rest of the vampires out of wit which got him a raised eyebrow from Raphael but he let it go as most of the vampires hated Camille.
Alec retrieved Simon but had Raphael take him home before he started working on Magnus' last courting gift.
So the next night, Magnus found Alec at his door like the first time he appeared but instead of a creature's body, he held on his hand a still beating heart of a vampire and he knew immediately who it belonged to.
That night Magnus and Alec officially said their 'I love you's and created together their engagment rings.
How things would go from here? It's up to the writer but maybe have Magnus be the one to kill Valentine and gift his head or heart to Alec in front of the whole Clave just to see their reactions(especially if they had gotten married in secret with a very bloody ritual and no one knew until Magnus called Alexander his husband as he gave him his gift).